It's the last version used to save a file that determines what can open it, not the first. If you converted hundreds of files from CC 2014 to CC 2015 (you have tried using the File > Open command from CC 2014 rather than double-clicking the files, right? That should work for files not converted) you'll probably want to use a script to do the IDML conversions. Peter Kahrel has a free one at Free InDesign scripts | Peter Kahrel
Thanks for the info.
But I only installed CC 2015 two days ago and now all the files on my server even going back to October 2015 and older are all now CC 2015 even though they were saved using CC 2014.
I didnt convert any of the files myself obviously that would be time consuming and pointless, for some weird reason CC 2015 has now said that all CC2014 files are now CC 2015
When I get info on the file from October which was saved using CC 2014 it now reads: Adobe InDesign CC 2015 Document.
It must be an overwrite on my local machine as the files themselves haven't been resaved only the Kind of doc has been changed.
Is there a preference or setting I have to delete from my library?
I tried choosing the Open with option and chose open with CC 2014 but it just says this file cannot be opened with an older version
Also if I have to resave all my indesign documents on the server they wont have the correct modified date , when searching for say a job i did in October 2015 they would all now read 4 feb 2016
Your OS thinks all files belong to the last version installed. Open CS 2014 first, then use the File menu to open the files.